FIG. 8 shows an antitheft system conventionally used in stores wherein commodities such as compact disk cassettes and magnetic tape cassettes are on display for sale, to protect these articles against shoplifting or like illegal acts.
The antitheft system comprises an antitheft gate 5 installed in the vicinity of an exit 8 of the store, and an alarm unit 4 attached to a commodity 7. The antitheft gate 5 has incorporated therein a circuit board 50 and a transmitting antenna (not shown). The circuit board 50 is provided with a transmitting circuit (not shown) for producing a notifying signal for the alarm unit 4.
With reference to FIG. 9, the alarm unit 4 comprises an electronic buzzer 41, circuit board 42, battery (not shown), etc. which are housed in a casing 40. The casing 40 has a surface formed with sound emitting holes 43 and an alarm actuating switch piece 44. The electronic buzzer 41 has its operation controlled by the circuit board 42 and is adapted to produce an alarm when the unit 4 is removed from the commodity 7 or when the unit 4 passes by the antitheft gate 5.
The commodity is placed on display in the store, with the alarm unit 4 attached thereto. When selling the commodity 7 to the customer, the clerk holds the buzzer 41 out of operation by sending a specified signal from a nullifying device (not shown) to the circuit board 42 of the alarm unit 4, then removes the unit 4 from the commodity 7 and hands the commodity to the customer in exchange for money.
If the customer wrongfully removes the alarm unit 4 from the commodity 7, the switch piece 44 is turned off to cause the buzzer 41 to produce a sound. Further if the customer acts to unlawfully bring the commodity 7 out of the store along with the alarm unit 4, the circuit board 42 of the alarm unit 4 receives a notifying signal from the transmitting antenna of the antitheft gate 5 to turn on the buzzer 41.
However, the antitheft system described has the problem of failing to prevent the theft of commodities reliably since the alarm is emitted only by the alarm unit 4.
Accordingly, the present applicant has already filed an application for patent on an antitheft system which is adapted to produce an alarm not only from an alarm unit but also from an antitheft gate in the event of an unlawful act so as to give improved reliability to the antitheft system (JP-A No. 279082/1996).
The antitheft gate for the antitheft system has a signal generator for producing a notifying signal for the alarm unit, and an alarm device capable of producing an alarm.
With reference to FIG. 10, the alarm unit 6 includes a receiving antenna 61 which comprises a coil L1 and a capacitor C3 connected in parallel therewith. The antenna 61 receives a signal, which is then fed to an incoming signal amplification circuit 62 for amplification and thereafter to a control circuit 63. When the signal from the amplification circuit 62 is a notifying signal from the antitheft gate, the control circuit 63 gives a drive command to an electronic buzzer 64, which in turn produces a sound in response to the command.
When the notifying signal is received from the gate via the amplification circuit 62, the control circuit 63 also delivers a signal preparation command to a signal processing circuit 65, which in turn prepares a second notifying signal. The second notifying signal is amplified in an outgoing signal amplification circuit 66, then fed to a transmitting antenna 67 comprising a coil L2 and a capacitor C4 connected in series therewith and sent out.
The alarm device in the antitheft gate produces an alarm on receiving the second notifying signal from the alarm unit 6.
The antitheft system described is so adapted that when the customer is about to wrongfully bring out the commodity from the store along with the alarm unit 6, an alarm is given not only by the unit 6 but also by the antitheft gate. In this way, an improvement is made for more reliable prevention of theft of commodities.
However, the alarm unit 6 of the antitheft system, which has the receiving antenna 61, circuits 62, 63 and electronic buzzer 64 for giving an alarm in response to the notifying signal from the antitheft gate, further comprises the circuits 65, 66 for preparing and processing the second notifying signal and the transmitting antenna 67 and therefore has the problem that the alarm unit 6 is large-sized and difficult to attach to small commodities.